1. Field of the Invention
The present invention is broadly concerned with strain sensor assemblies and methods, which are designed to be coupled with structures potentially subject to strains, in order to provide a wireless sensing of a strain threshold and/or progressive strain monitoring. More particularly, the invention is concerned with such assemblies and methods wherein a sensor element comprising a microwire having an amorphous or nanocrystalline metallic alloy core is applied to or imbedded in a structure such that the microwire core is placed in tension when the structure is subjected to strain. An induction detector separate from the structure is operable to interrogate the applied sensing microwire in order to induce a remagnetization response from the microwire core. The microwire core has a first remagnetization response of when the structure is unstrained, and a second, substantially greater, remagnetization response when the structure is strained.
2. Description of the Prior Art
Analysis and monitoring of stresses and strains plays a very important role in developing and maintaining engineered structures, such as bridges, buildings, or aircraft. Presently, the principal means of such monitoring is through the use of resistive strain gauges, in which a resister element made of fine conducting wire is secured to a support, which is in turn attached to a monitored structure. Structural deformations are transferred to the gauge wire, which proportionally changes the resistance thereof, and this change is monitored.
Increasingly, composite materials (e.g., carbon fiber-based composites) are used in constructing sophisticated structures, such as aircraft body components. It is very important to know the stress distributions inside of such composite parts, especially at areas such as glued connections, or the location of metal inserts. Conventional resistance-type strain gauges cannot be used in this context because the relatively large sizes thereof will create structural defects inside the composite parts. Another drawback of these resistive gauges is the necessity of having connecting wires or cables.
Attempts have been made in the past to devise miniature, essentially non-intrusive strain sensors with a wireless transfer of strain data. For example, WO2007/7054602 describes a multi-functional sensor device having a sensor made up of a multi-layer magnetic microwire consisting of a metal core surrounded by one or more outer layers, wherein at least the core or one of the outer layers is magnetic. The operation of the sensor relies on a magnetoelastic coupling between the magnetic layer of the sensor and the remainder of the layers. In order to detect strains in the surrounding structure, and consequently in the magnetic properties of the sensor, an AC current is passed through the metal core, picking up the output signal in the form of voltage, impedance, resistance, or inductance from the magnetic layer. This sensor still suffers from the problem of the necessity of wired connections to a monitoring device.
Sandacci et al., Stress Dependent Magnetoimpedance of Co-Amorphous Wires with Induced Axial Anisotropy for Tunable Microwave Composites, IEEE Transactions on Magnetics, Vol. 41, No. 10, October, 2005, pp. 3353-55, describe sensing microwires which may be incorporated into a dielectric matrix to provide wireless strain sensing via microwave interrogation. However, such microwave visualization is not usable with carbon fiber-based composites, because of the good electrical conductivity of the carbon filler.
Glass-coated amorphous sensing microwires have been used in the past in the context of electronic article surveillance (EAS) and authentication systems. Such sensing microwires, their production, magnetic properties, and behaviors, have been disclosed in the technical and patent literature. See, for example, U.S. Pat. Nos. 6,441,737 and 6,747,559; Horia Chirac, Preparation and Characterization of Glass Covered Magnetic Wires, Materials Science and Engineering A304-306, 166-71 (2001); Donald et al., The Preparation, Properties and Applications of Some Glass Coated Metal Filaments Prepared by the Taylor-Wire Process, Journal of Materials Science, 31, 1139-48 (1996); Wiesner and Schneider, Magnetic Properties of Amorphous Fe—P Alloys Containing Ga, Ge, and As, Phys. Stat. Sol. (a) 26, 71 (1974); and Antonenko et al, High Frequency Properties of Glass-Coated Microwires, Journal of Applied Physics, vol. 83, 6587-89. Continuous lengths of sensing microwires have been produced inexpensively by what is generally called in the art the Taylor process whereby either a pre-alloyed ingot or the required elemental constituents are melted in a generally vertically disposed glass tube that is sealed at the bottom. Once the alloy is converted to a molten state, using radio frequency (“rf”) heating for example, the softened bottom of the glass tube is grasped and drawn continuously. Rapid reduction of alloy cross-section, together with use of secondary cooling means, cause the alloy to become amorphous or nanocrystalline during drawing.
A typical sensing microwire may have a total diameter (both the wire core and glass coating) of several tens of microns. The alloy core and glass coating can be physically coupled to each other continuously or only at several spatially separated points. The glass-to-metal ratio, though variable, can be tightly controlled. For example, the typical thickness of a glass coating may be from about 1-5 microns for a 45-60 micron core diameter microwire, and typically 1-3 microns for 30 micron core diameter microwire. Sensing microwire elements for prior art EAS and authentication tags are usually cut to lengths ranging from 15 mm to 75 mm.
Prior art glass-coated amorphous sensing microwires produced by the Taylor method can be fabricated so as to exhibit very low coercivities (substantially less than 10 A/m), high relative permeabilities (substantially higher than 20000), substantially zero or slightly positive magnetostrictions, and large Barkhausen discontinuities (which means that the microwires exist essentially only in bimodal magnetic states). The remagnetization properties of sensing microwires are also important and can be adjusted based upon the makeup of the core alloy and the other physical parameters of the sensing microwires.
See also, U.S. Pat. Nos. 6,556,139; 4,134,538; 6,622,913; and 7,354,645; Published Application 2005/0109435; and Zukov et al., J. Mater. Res. 15, No. 10, October 2000.
There is accordingly a need in the art for improved strain sensors and methods which are very small in size so as to be useful for internal monitoring of structures, while also permitting wireless interrogation of the sensors without the need for electrical wires or cables, and being useful with essentially all types of structures including carbon fiber composites.